Introduction
Understanding how to apply cathodic protection (CP) current to reduce the potential difference between anodic and cathodic sites is fundamental for corrosion field technicians. Corrosion is driven by electrochemical cells formed on metal surfaces where anodic sites lose metal through oxidation, and cathodic sites undergo reduction reactions. The potential difference between these sites drives corrosion current, resulting in metal loss at anodic areas.
Cathodic protection works by supplying an external current to the metal structure, shifting its electrochemical potential negatively, and effectively making the entire structure behave as a cathode. This process eliminates or significantly reduces the potential difference between anodic and cathodic sites, slowing or stopping corrosion. This article provides a detailed and comprehensive explanation of the principles, mechanisms, and practical application of cathodic protection current to achieve this goal.
Understanding the Electrochemical Basis of Corrosion and Cathodic Protection
At its core, corrosion is an electrochemical process requiring four components:
Anode: The site on the metal surface where oxidation occurs, releasing metal ions and electrons.
Cathode: The site where reduction reactions consume electrons; no metal loss occurs here.
Electrolyte: A conductive medium such as soil or water that allows ionic movement between anode and cathode.
Metallic Path: The metal structure itself, providing an electrical connection for electron flow between anodic and cathodic sites.
Microscopic differences in metal composition, stress, or environment create anodic and cathodic sites on a single structure, establishing a corrosion cell. The potential difference between these sites drives the corrosion current, causing metal loss at anodic locations.
Cathodic protection interrupts this process by supplying electrons to the structure from an external source, shifting the structure’s potential negatively (polarization). When the structure’s cathodic sites are polarized to the potential of the most active anodic sites, the voltage difference driving corrosion current is eliminated, and corrosion stops.
Principle of Applying Cathodic Protection Current
Cathodic protection current is applied to the metal structure to:
Electronegatively polarize all cathodic sites to the potential of the most active anodic sites.
Reduce the potential difference between anodic and cathodic sites to zero.
Stop the flow of corrosion current and metal loss at anodic sites.
This is achieved by connecting the structure electrically to an external current source (either galvanic anodes or impressed current systems) and anodes placed in the electrolyte. The current flows from the anode through the electrolyte to the structure, supplying electrons that satisfy the cathodic reactions without requiring metal oxidation.
The external anode corrodes sacrificially or remains inert (depending on system type), completing the circuit and allowing continuous current flow.
Polarization and Potential Shifting
Polarization refers to the change in electrochemical potential of the metal surface caused by the applied current. As cathodic protection current increases, the structure’s potential shifts negatively in a stepwise manner:
Initially, the more positive cathodic sites polarize first.
As these sites reach the potential of the most active anodic sites, polarization spreads to other cathodic sites.
When all cathodic sites are polarized to the open circuit potential of the most active anodic sites, the potential difference is eliminated, and corrosion current ceases.
This polarization is measured as a shift in structure-to-electrolyte potential, typically using a copper-copper sulfate reference electrode (CSE). For steel in soil or water, a polarized potential of -850 mV vs. CSE is widely accepted as the criterion for adequate protection.
Current Requirements for Effective Cathodic Protection
The amount of cathodic protection current required depends on:
Surface Area to be Protected:
Larger exposed metal surfaces require more current.
Coatings reduce exposed area, lowering current needs to only coating defects (holidays).
Environmental Factors Affecting Polarization and Current:
pH: Acidic environments increase current requirements due to higher hydrogen ion concentration.
Temperature: Higher temperatures increase reaction rates, reducing polarization and increasing current needs.
Oxygen Levels: Higher oxygen availability reduces polarization, increasing current demand.
Electrolyte Movement: Flowing or turbulent electrolytes supply reactants faster, increasing current requirements.
Polarization Behavior:
The relationship between applied current and corrosion rate is logarithmic.
Initial increments of current cause the largest decrease in corrosion.
Overprotection (excessive current) can damage coatings or cause hydrogen embrittlement in susceptible metals.
Example Calculation: A bare 12-inch diameter, 5,000 ft steel pipeline buried in soil with 10,000 ohm-cm resistivity requires approximately 15.7 A of current for protection. If coated with 95% effectiveness (5% holidays), the current required drops to about 0.785 A.
Types of Cathodic Protection Systems and Current Application
Galvanic (Sacrificial) Anode Systems:
Use metals more active than the structure (magnesium, zinc, aluminum) as anodes.
Anodes corrode preferentially, supplying current to the structure.
Suitable for small current requirements and low-resistivity environments.
Limited driving voltage and current output.
Impressed Current Cathodic Protection (ICCP) Systems:
Use an external DC power source (rectifier) to supply current.
Anodes are inert or low-consumption materials (graphite, mixed metal oxide).
Suitable for large structures, high-resistivity soils, or poorly coated assets.
Adjustable current output allows precise control of polarization.
Measurement and Verification of Protection
To verify that cathodic protection current has effectively reduced the potential difference and halted corrosion:
Measure structure-to-electrolyte potential using a reference electrode (commonly copper-copper sulfate electrode, CSE).
Use instant-off potential measurements (taken immediately after interrupting CP current) to eliminate IR drop errors and obtain true polarized potentials.
Confirm that the polarized potential is at or more negative than -850 mV vs. CSE for steel structures.
Alternatively, verify a minimum of 100 mV polarization shift from the native (unprotected) potential.
Step-by-Step Procedure for Applying Cathodic Protection Current
Assess the Structure and Environment:
Determine the size, coating condition, and exposed metal surface area.
Measure soil or water resistivity, pH, temperature, and oxygen content.
Calculate Current Requirements:
Use surface area and environmental data to estimate current density needed.
Adjust for coating holidays and degradation over time.
Select Cathodic Protection System Type:
Choose galvanic anode system for small current needs and low-resistivity environments.
Choose impressed current system for large or poorly coated structures and high-resistivity soils.
Design Anode Placement and Configuration:
Position anodes to ensure even current distribution and minimize shielding effects.
Use distributed or remote anode configurations as appropriate.
Install System Components:
Install anodes with proper backfill to reduce ground resistance.
Connect wiring with insulated cables and secure, low-resistance joints.
Install rectifier (for ICCP) with appropriate sizing and protective features.
Apply and Adjust Current:
Energize the system and gradually increase current to achieve target polarized potential.
Monitor potentials and adjust current output to maintain protection criteria without overprotection.
Monitor and Maintain:
Perform regular potential surveys and system inspections.
Replace consumed anodes and repair wiring or coatings as needed.
Common Challenges
Inadequate Current: Insufficient current fails to fully polarize cathodic sites, allowing residual corrosion.
Overprotection: Excessive current causes coating disbondment, hydrogen embrittlement, or corrosion of amphoteric metals. Avoid polarized (instant-off) potentials more negative than -1,200 mV vs. CSE for pipelines.
Electrical Shielding: Barriers such as insulating materials, shorted casings, or reinforcing wires can prevent current from reaching some areas, causing localized corrosion.
Environmental Variability: Changes in soil moisture, temperature, or resistivity affect current distribution and protection effectiveness.
Measurement Errors: IR voltage drops and stray currents can distort potential readings; use instant-off measurements and proper electrode placement.
Summary and Key Takeaways
Corrosion is driven by potential differences between anodic and cathodic sites on metal surfaces.
Cathodic protection applies external current to polarize the entire structure cathodically, eliminating this potential difference and stopping corrosion.
The current required depends on exposed surface area, coating condition, and environmental factors such as pH, temperature, and oxygen content.
Two main CP system types exist: galvanic (sacrificial) anode and impressed current systems, each suited to different applications.
Protection effectiveness is verified by measuring structure-to-electrolyte potentials, aiming for -850 mV vs. CSE or a 100 mV polarization shift.
Proper design, installation, monitoring, and maintenance are essential to ensure effective and safe cathodic protection.
Awareness of common pitfalls such as overprotection, shielding, and measurement errors helps maintain system integrity.
By knowing these principles and procedures, corrosion field technicians can effectively apply cathodic protection current to reduce the potential difference between anodic and cathodic sites, thereby safeguarding critical infrastructure from corrosion damage.
Referenced Standards
NACE SP0169: Control of External Corrosion on Underground or Submerged Metallic Piping Systems.
ASTM Standards: Various for materials and testing.
ISO 15589-1: Cathodic Protection of Pipeline Transportation Systems.
AWWA D104: Automatically Controlled, Impressed Current Cathodic Protection for the Interior of Steel Water Tanks.